This disclosure relates, in various embodiments, to electrostatographic imaging members. The imaging members described herein are flexible electrostatographic imaging members which can be used as photosensitive members, photoreceptors or photoconductors useful in electrophotographic systems, including printers, copiers, other reproductive devices, and digital apparatuses. More particularly, the imaging members of this disclosure comprise an improved anti-curl back coating.
Flexible electrostatographic imaging members are well known in the art. Typical flexible electrostatographic imaging members include, for example: (1) electrophotographic imaging member belts (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring belt which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper. The flexible electrostatographic imaging members may be seamless or seamed belts. Typical electrophotographic imaging member belts include a charge transport layer and a charge generating layer on one side of a supporting substrate layer and an anti-curl back coating coated onto the opposite side of the substrate layer. An electrographic imaging member belt may, however, have a more simple material structure; it may have a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Although the scope of the present disclosure covers the preparation of all types of flexible electrostatographic imaging members, for reasons of simplicity, the discussion hereinafter will focus only on flexible electrophotographic imaging members.
Electrophotographic flexible imaging members may include a photoconductive layer including a single layer or composite layers. Since typical electrophotographic imaging members exhibit undesirable upward imaging member curling, an anti-curl back coating, applied to the backside, is required to balance the curl. Thus, the application of an anti-curl back coating is necessary to provide the appropriate imaging member with desirable flatness.
Electrophotographic imaging members, such as photoreceptors or photoconductors, typically include a photoconductive layer formed on a flexible electrically conductive substrate or formed on layers between the substrate and photoconductive layer. The photoconductive layer is an insulator in the dark, so that during machine imaging processes, electric charges are retained on its surface. Upon exposure to light, the charge is dissipated, and an image can be formed thereon, developed using a developer material, transferred to a copy substrate, and fused thereto to form a copy or print.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer. The charge transport layer is usually the last layer to be coated and is applied by solution coating followed by drying at elevated temperatures, then cooling to ambient room temperature. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the charge transport layer coating and drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support. Because the charge transport layer in a typical prior art photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer contracts more than the substrate support as it cools down to ambient room temperature. The resulting internal tension strain in the charge transport layer causes the photoreceptor to exhibit upward curling. If unrestrained, the photoreceptor would spontaneously curl upwardly into a 1.5-inch tube. To offset this curl and keep the photoreceptor web stock flat, an anti-curl back coating (ACBC) is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer.
The ACBC is typically applied to the imaging member by a solution coating process. Typical ACBC coating solutions generally contain a film forming polymer, a small amount of a polyester adhesion promoter, and an organic solvent(s), such as methylene chloride or a chlorinated solvent. After application of the coating solution to the back surface of the imaging member, the wet ACBC is dried at elevated temperatures to remove a substantial amount of the solvent to produce a solid layer. However, not all of the solvent may be removed from the ACBC during drying. For example, in forming a typical layer from a coating solution containing about 86 weight-% (wt-%) methylene chloride solvent and 14 wt-% dissolved solids, the solvent evaporates very quickly during the elevated temperature drying process. However, about 1.4 wt-% to 2 wt-% of the methylene chloride will generally still be present or trapped in the resulting ACBC (i.e., residual methylene chloride). The trapped solvent evaporates or “outgases” over time. The outgassed solvent, as vapor, can damage the charge transport layer either while it is in its original package or during cyclic belt function inside a machine cavity, causing premature charge transport layer cracking and shortening the imaging belt's service life.
Additionally, because the anti-curl back coating is on the backside of the substrate support, it directly interacts with the machine belt support rollers and backer bars; this causes substantial wear of the ACBC. This mechanical interaction has also been seen to promote ACBC static charge-up, producing substantial belt drive torque increases and, in some cases, resulting in absolute belt cyclic motion stalling. The wear on the anti-curl back coating also generates dust inside the machine cavity, causing undesirable dusty operating conditions. ACBC wear also reduces the thickness of the anti-curl layer, diminishing its ability to keep the photoreceptor belt flat.
Furthermore, under typical machine electrophotographic imaging conditions, ozone is constantly being generated by the charging device(s) and fills the machine cavity. This ozone exposure has been found to cause polymer chain scission in the ACBC, which weakens its mechanical property and exacerbates the ACBC wear problem.
In this regard, when the ACBC wears, its ability to reduce upward belt curling decreases. This upward belt curling, caused by loss of ACBC thickness, produces significant surface distance variations between the photoreceptor belt surface and the machine charging device. These variations cause non-uniform charging density over the photoreceptor belt surface, degrading copy printout quality. Thus, ACBC wear can cause visible defects in the printed image. When the imaging members curl upward, they may also physically interact/interfere with other xerographic subsystems; this will also lead to undesirable artifacts in a printed image.
Consequently, there continues to be a need for imaging members having an anti-curl back coating which contains little or no residual solvent, can reduce wear by suppressing or eliminating the effects of polymer chain scission and/or reducing surface friction between the ACBC and machine parts.